The present invention relates to a method of modifying certain organosiloxane copolymers, sometimes referred to as "MQ" resins. Such resins are condensation products comprising Q and M units, so designated employing the art-recognized convention for designating organosiloxane structural units in accordance with the number of oxygen atoms attached to silicon is employed herein. That convention uses the letters M, D, T and Q to designate said number of oxygen atoms as abbreviations for "mono", "di", "tri" and "quatro".
More particularly, the present invention relates to the use as a catalyst of a linear phosphonitrilic halide (LPNX) where X is a halogen atom and preferably chloro, a short chain linear phosphazene (SCLP) or a strong acid in contact with a mixture of an MQ resin and a functionalized organosilicon material. A reduction is effected in SiOH content, along with an exchange reaction between the MQ resin, whose core Q unit structure remains substantially intact during the reaction, and the functionalized organosilicon material having units such as H(CH.sub.3).sub.2 SiO--, CF.sub.3 CH.sub.2 CH.sub.2 (CH.sub.3).sub.2 SiO--, C.sub.6 H.sub.5 (CH.sub.3).sub.2 SiO-- or CH.sub.2 .dbd.CH(CH.sub.3).sub.2 SiO--.
Various techniques are constantly being evaluated for introducing functionality into MQ silicone resins and decreasing the proportion of by-products therein containing silanol groups. Some uses for functionalized MQ resins are as crosslinking agents and as reinforcing fillers in optically clear products. In addition, applications based on a deep section cure or liquid injection molding (LIM) often use functionalized MQ resins with a platinum catalyst. Pressure sensitive adhesives (PSA's) also can use a functionalized MQ resin in combination with a network Si--H and Si-vinyl package, employing a peroxide or platinum curing catalyst.
As shown in U.S. Pat. 5,319,040, substantially silanol-free MQ resin powder can be prepared by treating a organic solvent dispersion of a silanol-containing MQ resin with an organosilicon-nitrogen material, such as a silylamine or organosilazane. The resulting resin can be used in making heat-curable silicone compositions after it has been spray dried. The MQ resin can include M units having functional groups such as H(CH.sub.3).sub.2 SiO-- and CH.sub.2 .dbd.CH(CH.sub.3).sub.2 SiO--.
U.S. Pat. 4,585,836, directed to a PSA, uses a silanol-containing resin copolymer having M and Q units. The copolymer can be made by treating a silica hydrosol at a low pH with a triorganosiloxy unit source, which can be a disiloxane or chlorosilane having functional groups. U.S. Pat. 4,584,355 shows the use of a functionalized organonitrogen end-blocking agent, such as a silazane, to reduce the level of silanol while introducing functional groups into a silanol-containing MQ resin.
Although various procedures are available for decreasing silanol proportion in and introducing functionality into MQ resins, methods involving the direct treatment of a silica hydrosol at a low pH with a triorganosiloxy unit source are economically unattractive, since considerable loss of triorganosiloxy units can occur during the hydrolysis step. Introduction of functional groups using a functionalized organonitrogen end-blocking agent also are somewhat limited, since they are based on the silanol content of the MQ resin.
In view of the strong demand for functionalized MQ resins, additional procedures for making such resins are constantly being evaluated.
The present invention is based on the discovery that functionalized MQ resins can be made by effecting an exchange reaction between triorganosiloxy groups free of functionality, such as trimethylsiloxy, on the surface of a preformed MQ resin, and certain functionalized organosilicon materials selected from organohalosilanes and organosiloxanes, such as disiloxanes. One feature of the invention is that a substantial reduction in the level of SiOH groups on the MQ resin surface can occur simultaneously with such exchange. Further, exchange can be facilitated without penetrating the MQ resin core structure by using an effective amount of a catalyst such as a LPNX, a SCLP or a strong acid such as trifluoromethanesulfonic acid (triflic acid).